1. Field of the Invention
The present invention relates to improvements in methods and apparatus for making man-made snow. More particularly, it relates to a snow-making apparatus and method in which "external mix" nucleators are used to inject tiny ice particles or "nuclei" into a fine spray of water to convert the water particles to snow particles or crystals before descending to earth.
2. Discussion of the Prior Art
Many different types of apparatus have been devised and used for producing "man-made" snow. Typically, such apparatus is found at ski resorts and operates to supplement the supply of natural snow on ski trails and surrounding areas. Virtually all types of snow-making devices produce snow by projecting water droplets into a stream of cold air, the latter serving to cool the droplets to a temperature at which they convert to ice crystals before descending to the ground. Some devices, known as "fan guns," employ a large motor-driven fan for creating the cooling air stream. In other devices, known as "snow canons" or "snow guns", the air stream is provided by a source of compressed air. The cooling air stream acts to enhance the water-to-snow conversion efficiency of the device by (a) creating a turbulent air flow which assists in both the droplet cooling and mixing processes, and (b) lengthening the droplet flight time or "hang time", thereby giving the droplets more time to cool and crystallize before reaching the ground.
In U.S. Pat. No. 4,711,395 issued to Louis Handfield, there is disclosed a fan gun of the type mentioned above. This fan gun is of the "central nozzle" variety in that the water droplets are introduced into the fan-produced air stream by a water nozzle located along the central axis of a barrel-shaped fan housing through which the air stream is propelled by the motor-driven fan. The water nozzle disclosed in this patent is of the type used on the hoses of fire-fighting equipment. Its output is adjustable to provide a desired throughput and spray pattern, and it includes spinning turbine teeth which act to break up the water supplied thereto into droplets of a "size ideal for snow-making". In the art, this phrase is understood to mean that the droplets are about 500-1000 microns in size because, in the case of a water nozzle of the type disclosed, i.e., the "Turbojet" (trademark) nozzle made by Akron Brass Company, the nozzle is not capable of breaking up the discharged water into droplets or particles any finer. To facilitate the conversion of such water droplets to ice crystals by the fan-produced air stream, a plurality of "nucleators" are arranged about the water nozzle and within the barrel-shaped fan housing. Each of the nucleators comprises a nozzle connected to a source of water. The nucleator nozzles act to atomize the water provided thereto to produce tiny water particles (e.g. 10 microns in size) called "nuclei," and to inject such water nuclei into the swirling water/air mixture provided by the water nozzle and fan combination. Owing to their small size, the nuclei freeze first and thereby act as seeds for the further formation of ice crystals in the water/air mixture.
Depending on ambient conditions, most commercially available fan guns are advantageous in that they are capable of converting relatively large volumes of water to snow per unit time. For example, at a temperature of about 15 degrees F. (-9 degrees C.), most fan guns are capable of converting between 75 and 100 gallons of water per minute to snow. But fan guns are generally considered disadvantageous from the standpoints of cost and size. More specifically, they are costly to manufacture and, owing to the motorized fan component, require considerable electrical power to operate. Also, due to their physically large size (typically, between 18 and 36 inches in diameter and having a weight of between 1000 and 2000 lbs.), fan guns tend to be difficult to manipulate in order to produce snow where desired, e.g., along narrow ski trails and other difficult to reach places. Further, owing to their large size, they are awkward, at best, to support, manipulate and operate at elevated positions, such as on towers or the like. This is especially true in windy conditions. As indicated above, placement of any snow-making device at an elevated position, and in particular more than about 15 feet above ground level, has a dramatic effect on the water-to-snow conversion efficiency of the device owing to the increase in droplet flight time and, hence, the cooling time of the droplets.
There are many smaller and less costly alternatives to the fan guns discussed above, including the air/water snow guns disclosed in the commonly assigned U.S. Pat. No. 3,829,013 issued to H. R. Ratnik, and in U.S. Pat. No. 4,199,103 issued to H. K. Dupre. Rather than employing a motorized fan to effect droplet cooling, both of these snow guns use a source of compressed air to cool the droplets. In the Ratnik device, water droplets are formed in an enclosed housing before being propelled into the atmosphere by the compressed air. In the Dupre snow gun, a stream of water is sprayed into the atmosphere and a jet of compressed air, located downstream of the water spray, is used to both break up the water into small particles and convert such particles to ice crystals. While being considerably less expensive to manufacture and operate, these snow guns are generally incapable of producing the volume of snow provided by fan guns.
In the commonly assigned International Patent Application No. WO 96/35087, published on 7 Nov. 1996 in the names of H. R. Ratnik and T. C. Wang, there is disclosed a fanless snow gun which minimizes many of the above-noted problems. This snow gun comprises the combination of one or more bulk water nozzles for projecting a relatively fine spray of water particles into the air, each of the particles having an average size smaller than about 300 microns; and a plurality (e.g., from 2 to 6) of nucleating nozzles which are radially spaced about the water nozzle for injecting ice particles or "ice nuclei" into the spray of water particles to provide nucleation centers about which the water particles freeze and form ice crystals. Preferably, the snow gun is supported by a tower high above ground level (e.g., 6 to 9 meters above) to enable sufficient flight time for all of the water particles in the spray to collide with the ice nuclei and thereby freeze into snow crystals before reaching the ground. Each of the nucleating nozzles comprises a housing in which water and compressed air are internally mixed to produce the ice nuclei. As noted in this publication, each of the nucleating nozzles optionally includes an internal electric heating coil which serves to prevent the nucleator nozzle from "freezing up" at ambient temperatures below freezing. This freezing up is apt to occur each time the nucleator is shut off as residual water trapped in the nozzle contacts the cold air within the housing. In sufficiently cold ambient conditions, freezing can occur even when the nucleator is operating. As a practical matter, such heaters are always required in snow guns of this type to assure that snow can be made at virtually any temperature below freezing. Thus, the need for such heaters in such "internal mix" nucleators adds manufacturing costs to the product and should be avoided if possible. Moreover, the need for heaters imposes a requirement for electrical power to be available at each snow-making site. A further disadvantage of such "internal mix" nucleators is that, if the water pressure applied to the nucleator housing rises above a certain level, the water pressure within the housing can choke or interrupt the air flow, thereby causing the production of nuclei to stop or be intermittent. In typical ski areas where such snow-making equipment is most often used, it is not uncommon to experience the type of water pressure changes that give rise to this effect.